Maturation of thymocytes is a selective process in which only a small percentage of the developing T cells survive. We propose to analyze T cells with known antigen specificity as they develop. By isolating and characterizing these cells at specific stages of development we anticipate being able to identify known and novel genes involved in this complex process. The novel and powerful system that we will develop will give us the ability to identify and isolate cells just prior to and just post positive selection. Our initial work will focus on the complete characterization of two novel thymocyte populations we have recently identified. Preliminary work with these thymocyte populations suggests that positive selection and the differentiation that leads to lineage commitment are closely related processes. Next, building on our previous work, we will develop reagents that will allow us to directly assess the developmental potential of the thymocytes within specific populations. Finally, we will examine the extent of the limitations that positive selection on self-peptide:MHC complexes imposes on the specificity of the mature TCR repertoire. By taking advantage of our recent work concerning the function of the protein HLA-DO, we will modify the antigen processing capabilities of the cortical thymic epithelial cells that mediate positive selection. The presence of HLA-DO is anticipated to alter the intrathymic self-peptide repertoire. We then will then ask if this altered intrathymic self-peptide repertoire fundamentally changes the specificity of the mature T cell repertoire. This work will give us insights into phenomenon such as immunodominance and the link between certain MHC alleles and autoimmunity.